In an ordinary small motor, the outer circumferential surface of its motor casing has a circular shape (round shape). When such a round motor is to be mounted in an apparatus or on a wiring board, a whirl-stop must be employed for preventing rotation of the motor. When the round motor is mounted in an apparatus, the round shape tends to be accompanied by wasted space within the apparatus. Thus, in view of prevention of rotation of a motor in relation to a mounting surface and space efficiency, there is known impartment of a quadrangular or higher polygonal external shape to a motor.
Meanwhile, there has been known a motor which has a quadrangular external shape and in which a quadrangular magnet is used unlike the case of the above-descried round motor (Patent Documents 1, 2, and 3). The quadrangular external shape of the motor prevents rotation of the motor in relation to a mounting surface and enhances the space efficiency. However, in the case of a motor which has a quadrangular external shape and in which a quadrangular magnet is used, the motor casing has flat wall portions, which generate large vibrations, thereby causing a problem of vibration and noise.
FIG. 9 is a sectional view showing a motor which is described in Patent Document 3 and in which the magnetic poles are disposed at the respective corners between the side portions of the motor casing. In FIG. 9, the motor casing (yoke) has a quadrangular section and accommodates therein a 4-pole field magnet magnetized with alternating N and S poles. This field magnet is magnetized such that the centers of magnetic poles are located at the respective corners between the side portions of the yoke. The motor in which magnetic poles are disposed at the respective corners between the side portions not only enables the magnet to be formed in a smaller size and be efficiently disposed, but also can reduce cogging torque. However, since the motor casing of the motor has flat wall portions of a fixed thickness, the motor has a problem in that the flat wall portions vibrate and generate a large noise.
In order to improve productivity, desirably, the field magnet of a small motor is manufactured separately from the motor casing (yoke) and is then assembled with the yoke. Usually, such assembly employs a press-fit technique. However, in actuality, press-fitting cannot be performed in such a manner that the entire outer circumferential surface of the magnet uniformly comes into contact with the inner circumferential surface of the motor casing. Therefore, in the motor structure as shown in FIG. 9, the corner portions of the magnet are bought into close contact with the inner circumferential surface of the motor casing, particularly, at the apexes of magnetic poles of the magnet at which the magnetic field intensity must be increased. Therefore, there must be employed a design method in which corner portions of the magnet are fixed by means of press-fitting and a slight clearance is provided between each of the flat portions of the magnet and the motor casing. When the structure in which the corner portions of the magnet are fixed by means of press-fitting is employed, vibrations of the flat wall portions become smaller than those in the case where the magnet is not press-fitted. However, the vibration reducing effect is still insufficient in applications in which more reduction of vibrations is required.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. H7-59322
[Patent Document 2] Japanese Utility Model Application Laid-Open (kokai) No. S64-12455
[Patent Document 3] Japanese Patent Application Laid-Open (kokai) No. 2007-6688